NIH has just released three Funding Opportunity Announcements (FOAs) for supplements to existing grant awards to be made using stimulus funds. They are for competitive revision, general administrative supplement, and administrative supplement for student summer research experiences.
Competitive revisions are to support "a significant expansion of the scope or research protocol of approved and funded projects", are to be reviewed by the study section that reviewed the parent grant, and are due on April 21. Administrative supplements are to support activities "within the general scope of the peer-reviewed activities and aims approved within the parent grant", are only reviewed administrative by program staff of the funding institute of the parent grant, and are subject to due dates established by the funding institute.
Not all institutes are participating in all three of these mechanisms, and will be establishing their own standards and procedures for what kinds of things they want to fund using them. This Web page contains links to each institute's policies for stimulus supplements.UPDATE: I had a nice chat with one of my program officers about this today. Reading between the lines, I think that you have to be fucking nuts to submit for a competitive revision rather than an administrative supplement, *unless* there is absolutely no way you can think of something to propose that is even arguably "within the general scope" of the parent grant. Reasons for thinking this:
(1) The decision whether the science proposed in a supplement request is "within the general scope" is completely within the discretion of program staff.
(2) Some institutes have forbidden requests for competitive revisions.
(3) The administrative and peer review burden for competitive revisions is *vastly* greater than for administrative supplements, *both* for applicants and for NIH. This is because competitive revisions need to be submitted using Grants.gov (if the parent was electronic) and handled by CSR receipt and referral, and then reviewed by study sections, either at CSR or inside institutes, depending on where the parent application was reviewed. Administrative supplements are to be submitted by e-mail to program staff.
(4) The administrative supplement deadline for 2009 funding is going to be substantially later than the April 21 competitive revision deadline. (UPDATED AGAIN: It appears that for at least some institutes, the deadline for administrative supplement applications for 2009 funds is even earlier than the competitive revision deadline!)
(5) Administrative supplement requests are going to be funded right the fuck away, and not much later, as will competitive revisions.
All of these factors make it very clear that both the institutes and applicants will benefit from there being as few competitive revision applications and as many administrative supplement applications as possible.

Since the submission dates, eligibility criteria, budget allowance, etc. for the administrative & summer research experience supplements vary by IC (some pre-dating the competitive revision deadline of April 21), I'm adding deadlines and links to supplement request details on my ARRA Resources Page for each IC as they come online.

If you're a new investigator without a funded grant eligible for a stimulus administrative add-on you're an idiot if you're not talking to the funded people around you right now figuring out how you can help them get supplements that can help you. At least that's the only way I can figure out right now on how to get into this game.

It would be wholly within the explicit goals of the stimulus to include grad student and post-doc stipends in the requested budget for a supplement or revision. In fact, one would be foolish not to include such lines in the requested budget and to make the explicit assertion that this will enable the appointment of new, or retention of existing, trainees.

Still think the easiest answer for institutes like NIGMS would be to restore administrative across the board cuts to grants for the next two years. Minimal work for everyone, good payout either buying equipment or retaining talent who haven't been able to find jobs do to cutbacks at universities and companies. I am guessing the supplements for equipment and talented postdocs will be the low hanging fruit that many will go for.